Three terminal active devices, such as bipolar junction transistors (BJTs), field-effect transistors (FETs), metal-oxide-semiconductor field-effect transistors (MOSFETs), heterojunction bipolar transistors (HBTs), and high-electron-mobility transistors (HEMTs), operate with the common feature that the voltage gain of the device has a negative sign. That is, an increase in voltage on the control terminal results in a decreasing voltage in the output terminal. For a device with voltage gain having a negative sign at the output terminal, the voltage gain having a negative sign causes the device's Miller capacitance occurring between the gate and drain (or base and collector, etc.) to be amplified. This amplification of the device's Miller capacitance contributes to the frequency related roll-off of device gain.
Miller capacitance generically refers to the parasitic capacitance between the output terminal and control terminal of an active electronic device. Specifically, it refers to the capacitance between base and collector in a BJT, grid and plate in a vacuum tube, and drain and gate in a JFET, MOSFET or HEMT. Miller capacitance is intrinsic to the device, and is a function of the device's geometry, composition, and instantaneous operating voltage.
When an active electronic device is configured as a voltage amplifier, which operates with a grounded emitter, a cathode, or a source, the active electronic device typically exhibits voltage gain with magnitude greater than one but with a negative sign. Consequently, the parasitic Miller capacitance between control node and output acts as a negative feedback element. If the voltage on the control terminal of the active device rises, the output voltage falls and results in current flow within this Miller capacitance. This effect is amplified by the voltage amplification of the device. The current within the Miller capacitance results in a decreasing gain of the device at higher frequencies due to the increasing current diverted from the control terminal into the Miller capacitance. This current is superimposed onto the gate current required to charge and discharge the gate-source capacitance.
Efforts to design amplifier devices with improved high-frequency performance have generally involved means to reduce Miller capacitance. For example, for a a pentode or tetrode vacuum tube, a shielding screen can be interposed between the plate and control grid of the vacuum tube to block most of the effects of the plate voltage from the grid. Cascode amplifiers behave analogously in solid-state devices.
All currently known active devices possess voltage gain with negative sign and exhibit Miller capacitance.
A recently developed alternative active device is the Piezoelectric Transistor (PET). A PET includes a piezoelectric actuator (sometimes referred to as a piezoelectric transducer) coupled to a piezoresistive element (e.g., piezoresistive switch) to create an electromagnetic analogue to a conventional transistor.